Forensic Pseudoscience

For those of you who may have recently installed security updates on Windows 10 workstations in the past few days, you may notice that you receive a peculiar error when trying to establish a remote desktop connection to a server that worked prior to installing the updates. The initial March 13, 2018, release updates the CredSSP authentication protocol and the Remote Desktop clients for all affected platforms. The CVE-2018-0886 consists of installing the update on all eligible client and server operating systems and then using Group Policy or registry settings to configure the options on both clients and servers. Let’s take a look at Windows 10 RDP CredSSP encryption oracle remediation error fix.

Windows 10 RDP CredSSP Encryption Oracle Remediation Error Fix

Just a couple of days ago, the cumulative updates were released below for Windows 10 and Server 2016, etc.  These cumulative updates include the fix for the CredSSP encryption vulnerability.

May 8, 2018 – KB4103721 (OS Build 1803)
May 8, 2018 – KB4103727 (OS Build 1709)
May 8, 2018 – KB4103731 (OS Build 1703)
May 8, 2018 – KB4103723 (OS Build 1609 & Server 2016)

Once you have installed the patch on a “vulnerable” workstation and attempt to connect to an unpatched server, you will see the following error message that happens after you type in your password to authenticate to the RDP session.

CredSSP authentication error after installing May 8 2018 patch Windows 10

There is a local policy setting that is added with the installed security updates.  You can find this at Computer Configuration >> Administrative Templates >> System >> Credentials Delegation >> Encryption Oracle Remediation.  By default this is set to Not configured.

Windows 10 RDP CredSSP encryption oracle remediation error Fix

To Fix the issue as a workaround, set the policy to Enabled and set the Protection Level to Vulnerable.  ***Note*** – This is not recommended by Microsoft, as making sure both the client and server is patched is best practice.  However, setting the policy to Vulnerable allows your workstation to now connect to the remote desktop session that was previously blocked by the mitigation.

Settings contained in the Encryption Oracle Remediation Fix

CredSSP Encryption Oracle Remediation Policy Settings

There are three settings contained in the policy setting that can be enabled.

Force Updated Clients: Client applications which use CredSSP will not be able to fall back to the insecure versions and services using CredSSP will not accept unpatched clients. Note: this setting should not be deployed until all remote hosts support the newest version.

Mitigated: Client applications which use CredSSP will not be able to fall back to the insecure version but services using CredSSP will accept unpatched clients. See the link below for important information about the risk posed by remaining unpatched clients.

Vulnerable: Client applications which use CredSSP will expose the remote servers to attacks by supporting fall back to the insecure versions and services using CredSSP will accept unpatched clients.

CredSSP Encryption Oracle Remediation Registry Setting

Alternatively, you can set this policy setting via the registry and a reboot.

• [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System\CredSSP\Parameters] “AllowEncryptionOracle”=dword:00000002

CredSSP Encryption Oracle Remediation Group Policy

These settings can be applied via a Group Policy Setting.  For a great walk through of how to do this, check out this post here: https://www.mcbsys.com/blog/2018/03/updating-the-credssp-group-policy/

Takeaways

Patching is becoming ever more important with security vulnerabilities that are present today.  Security is on the minds of everyone and it should be.  Compromised systems can lead to data loss and data leak.  Keeping up with Microsoft patches and having a routine schedule of patch application is essential for enterprise datacenters running Microsoft server operating systems.  By mitigating known vulnerabilities the attack surface is drastically reduced and attacks become more difficult for the would be attacker.